Non-functioing Gauges

Last weekend I purchased a 1973 Silverline Comoro 19V with a 165HP Mercruiser I/O in very good condition.
Yesterday I took her out on the lake for the first time. As the PO hadn't run the boat in over a year, it ran very well. However, with the exception of the temp gauge, which is a replacement, none of the original gauges work; oil press, amp, trim, fuel, and tach.
I did a quick visual under the dash and all the connections are tight and clean.
Where would I start to troubleshoot this issue?

Re: Non-functioing Gauges

1stgen, take 'em one at a time. There's probably something common to the non-working gauges (power, ground, etc.) Pick a gauge and trace it back to the sending unit and - on the other end - to the negative ("ground"). When you find what's wrong with that one, you'll likely have the cause of the problems with the others.

See the two attachments.

Re: Non-functioing Gauges

Gauges are powered from the "I" terminal on the ignition switch. That wire feeds the first gauge in the string from which the others are all daisy chained. The same principle applies to the ground connection. Sender signals siimply do not all fail at the same time since they are fully independent of each other. so you are looking for a power or ground issue. A voltmeter is your friend here. If you don't have one or know how to use one, they are a $15.00 item at any home imiprovement store and they come with instructions. If you check the stickies at the very top of this forum (not this page) you will find a "Generic Boat Wiring Diagram" that contains typical gauge wiring..

Re: Non-functioing Gauges

His may help.

View attachment 165 wire.pdf